linux-kernel - Re: [PATCH v1 1/4] include/linux/mmzone.h: add documentation for pfn

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Message-ID: <f900f889-f059-7d48-dfc7-145c8cd54cf6@redhat.com>
Date:   Tue, 20 Apr 2021 11:22:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/4] include/linux/mmzone.h: add documentation for
 pfn_valid()

On 20.04.21 11:09, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> Add comment describing the semantics of pfn_valid() that clarifies that
> pfn_valid() only checks for availability of a memory map entry (i.e. struct
> page) for a PFN rather than availability of usable memory backing that PFN.
> 
> The most "generic" version of pfn_valid() used by the configurations with
> SPARSEMEM enabled resides in include/linux/mmzone.h so this is the most
> suitable place for documentation about semantics of pfn_valid().
> 
> Suggested-by: Anshuman Khandual <anshuman.khandual@....com>
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>   include/linux/mmzone.h | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 47946cec7584..961f0eeefb62 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1410,6 +1410,17 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>   #endif
>   
>   #ifndef CONFIG_HAVE_ARCH_PFN_VALID
> +/**
> + * pfn_valid - check if there is a valid memory map entry for a PFN
> + * @pfn: the page frame number to check
> + *
> + * Check if there is a valid memory map entry aka struct page for the @pfn.
> + * Note, that availability of the memory map entry does not imply that
> + * there is actual usable memory at that @pfn. The struct page may
> + * represent a hole or an unusable page frame.
> + *
> + * Return: 1 for PFNs that have memory map entries and 0 otherwise
> + */
>   static inline int pfn_valid(unsigned long pfn)
>   {
>   	struct mem_section *ms;
> 

I'd rephrase all "there is a valid memory map" to "there is a memory 
map" and add "pfn_valid() does to indicate whether the memory map as 
actually initialized -- see pfn_to_online_page()."

pfn_valid() means that we can do a pfn_to_page() and don't get a fault 
when accessing the "struct page". It doesn't state anything about the 
content.

-- 
Thanks,

David / dhildenb